The synthesis of diamond crystals by high pressure, high temperature processes has become well established commercially. Preferred methods for making diamonds are disclosed and claimed in U.S. Pat. Nos. 2,947,610--Hall et al and 2,947,609--Strong. Apparatus for the conduct of such processes is described and claimed in U.S. Pat. No. 2,941,248--Hall. The Hall et al, Strong and Hall patents are incorporated by reference.
Diamond growth in the aforementioned processes occurs by the diffusion of carbon through a thin metallic film of any of a series of specific catalyst-solvent materials. Although such processes are vey successfully employed for the commercial production of industrial diamond, the ultimate crystal size of such diamond growth is limited by the fact that the carbon flux across the catalyst film is established by the solubility difference between graphite (the typical starting material) and the diamond being formed. This solubility difference is generally susceptible to significant decrease over any extended period due to a decrease in pressure in the system and/or poisoning effects in the graphite being converted.
On the other hand, in the method of growing diamond on a diamond seed crystal disclosed in U.S. Pat. No. 3,297,407--Wentorf, Jr. (incorporated by reference) a difference in temperature between the diamond seed and the source of carbon is relied upon to establish a concentration gradient in carbon for deposition on the seed. Catalyst-solvents disclosed in the aforementioned Hall et al and Strong patents are used in the temperature gradient method as well. The growth of diamond on the seed material is driven by the difference in solubility of diamond in the molten catalyst-solvent metal at the nutrient (source of carbon) and at the seed, between which locations a temperature gradient exists. Most important, this general type of reaction vessel configuration presents a pressure stable system so that pressure can more readily be kept in the diamond stable region.
By very carefully adjusting pressure and temperature and utilizing relatively small temperature gradients with extended (relative to growth times for thin film method) growth times, larger diamonds can be produced by the method as taught in the Wentorf patent than by the thin-film method.
Attempts to reliably produce very high quality diamond growth, however, have presented a number of apparently mutually exclusive, yet simultaneously occurring problems. These problems are:
(a) the strong tendency for spontaneous nucleation of diamond crystals near the diamond seed material (which occurs with increase in the temperature gradient over the "safe" value); if the growth period is extended to produce diamond growth from the seed of greater than about 1/20 carat in size the nucleated growth competes with the growth from the diamond seed with subsequently occurring collisions of multiple crystals that result in stress fractures therein, and
(b) either partial or complete dissolution of the diamond seed material in the melted catalyst-solvent metal during that part of the process in which the catalyst-solvent medium becomes saturated with carbon from the nutrient source and then melts; such dissolution produces uncoordinated diamond growth proceeding from spaced loci, which growths upon meeting, result in subsequent confused, flaw-filled growth.
In addition to overcoming the problems of spontaneous nucleation of diamond and diamond seed dissolution, it is highly desirable to be able to exercise reproducible control over the diamond growth process and, thereby, be able to produce novel diamond products, e.g. diamonds having unique color patterns and characteristics as well as affording the possibility of optimizing one or more physical properties in a given diamond.
In the context of this invention the following words have the meanings set forth:
(a) dopant: an impurity which, if present at the site of growing diamond, will enter the growing diamond lattice and influence the physical, mechanical and/or electrical properties of the diamond growth;
(b) getter: a material the atoms of which, if present at the site of growing diamond, will prevent or limit the entry of one or more dopant materials into the developing diamond growth, and
(c) compensator: a material the atoms of which, if present at the site of growing diamond will enter the growing diamond lattice and partially or completely offset the usual influence of one or more dopant materials present in the lattice with respect to physical, mechanical and/or electrical properties of the diamond.